Hydrocarbon combustion in diesel engines, stationary gas turbines, and other systems generates exhaust gas that must be treated to remove nitrogen oxides (NOx), including NO, NO2, and N2O. The exhaust generated in lean-burn engines is generally oxidative, and the NOx needs to be reduced selectively with a heterogeneous catalyst and a reductant, which is typically ammonia or a short-chain hydrocarbon. The process, known as selective catalytic reduction (SCR), has been thoroughly investigated.
Various combinations of ceria and Group 5 metals (V, Nb, Ta) have been used for SCR and other catalytic processes. Usually, at least 10 wt. % of the Group 5 metal or Group 5 metal oxide is present in the catalyst, as is evident from a review of related academic publications.
Le Gal et al. (J. Phys. Chem. C 116 (2012) 13516), for instance, teach doping of ceria with 10 to 50 atom % of tantalum. The catalysts are made by co-precipitating hydroxides from cerium nitrate and tantalum(V) chloride precursors. The authors concluded that tantalum substitutes for cerium in the normal fluorite structure of ceria, as indicated by contraction of the lattice parameter with increasing amounts of Ta. The catalysts are used for water splitting during solar thermochemical hydrogen generation.
S. Zhao et al. (Appl. Catal. A 248 (2003) 9) studied the effect of oxide dopants in ceria on n-butane oxidation. Catalysts having 10 at. % of Nb or Ta were prepared using a sol-gel method by reacting cerium(III) nitrate hexahydrate with niobium(V) chloride or tantalum(V) chloride. After dissolving the salts in water, the solutions were dried and calcined. The catalysts were not used for selective catalytic reduction.
K. Yashiro et al. (Solid State Ionics 175 (2004) 341) prepared “niobia-doped cerias” having less than 1 at. % of niobium and measured their electrical conductivity. The title is a misnomer because the ceria is actually prepared in the presence of preformed niobia. In particular, an aqueous solution of cerium(III) nitrate is combined with a mixture of oxalic acid and well-dispersed Nb2O5 powder. Cerium oxalate precipitates with dispersed particles of niobia. The precursor is then calcined at 1400° C. The product is not used for an SCR process.
E. Ramirez-Cabrera et al. (Solid State Ionics 136-137 (2000) 825) teach niobia-doped cerias and their use to convert methane to synthesis gas. The doped cerias contained 0.7 or 2.5 mole % of Nb2O5 and were made by co-precipitation of cerium and niobium hydroxides, followed by calcination.
In a series of papers (Appl. Catal. B 103 (2011) 79; Appl. Catal. B 88 (2009) 413; and J. Phys. Chem. C 114 (2010) 9791), M. Casapu et al. describe niobia-ceria catalysts and their use for SCR and soot oxidation. The catalysts tested contained 10 wt. % Nb2O5 on a mixed ceria-zirconia support or 30 wt. % Nb2O5 on ceria. The catalysts were made by co-precipitation or by wet mixing ammonium niobate oxalate with ceria. Interestingly, these catalysts become less active for NOx reduction upon calcination or hydrothermal aging at elevated temperatures (see, e.g., FIG. 9 (a) and (b) in Appl. Catal. B 103 (2011) 79), i.e., the opposite of what we found.
Combinations of niobia and ceria are also discussed in patents and published patent applications. As with the papers discussed in the preceding paragraph, EP 2368628 describes catalysts comprising at least 10 wt. % ceria and at least 10 wt. % of niobia and their use for an SCR process. The catalysts are prepared by co-precipitation or wet mechanical mixing, followed by calcination at elevated temperature (550° C. or 800° C.). In the latter case, ammonium niobate(V) oxalate hydrate is combined with ceria, and the resulting slurry is dried overnight at 80° C. and is thereafter calcined at 550° C. As shown in FIGS. 5 and 7, these catalysts become much less active when they are calcined for 12 hours at 800° C.
U.S. Publ. No. 2013/0121902 teaches mixed oxides of ceria, zirconia, niobia, and a rare earth sesquioxide as catalysts for an SCR process. Addition of niobia to the commercially available catalyst from ceria, zirconia, and the rare earth oxide is said to improve aging stability. The catalysts generally have high levels of zirconia. In one example with a low percentage of niobia (3 wt. %), the catalyst has 43% zirconia and 9% Nd2O3.
U.S. Pat. No. 6,605,264 teaches niobium containing zirconium-cerium based solid solutions and their use as “high oxygen ion conducting” or oxygen storage materials. The solid solutions comprise “up to about 95 mole % zirconium, up to about 50 mole % cerium, about 0.5 to 15 mole % rare earth metal(s) and about 0.5 to about 15 mole % niobium.” All of the examples have more than 50 mole % zirconium.
Catalysts comprising niobia and ceria and their use for SCR applications are disclosed in a series of international applications (see PCT Int. Appl. Nos. WO 2012/041921, WO 2012/004263, and WO 2013/037507). In the '263 publication, the proportion of niobium oxide to cerium oxide is 2 to 20%. Catalysts having <50% Zr are shown to have greater capability for reducing hydrogen compared with a similar catalyst made with 77.6% Zr. In two examples (Exs. 9 and 10), 3.2 wt. % or 8.6 wt. % of Nb2O5 is present, zirconia is omitted, and the balance is ceria. However, these catalysts are essentially ceria-encapsulated niobias rather than niobia “doped on” ceria. As shown in the examples, the catalysts are made by forming ceria in the presence of a smaller proportion of pre-formed niobia. Our own work (described herein) demonstrates that at identical proportions of niobia and ceria, these catalysts are less effective for NOx reduction than compositions in which the niobia is doped on ceria. Moreover, as shown in the '921 publication (Table 5, Exs. 9 and 10), these catalysts also appear to deactivate upon hydrothermal aging (750° C., 16 h).
New emission limits for diesel vehicles target both NOx and particulates. Commonly, this requires the use of two separate systems: an NH3-SCR system to remove NOx and a catalytic soot filter (see, e.g., U.S. Pat. No. 4,902,487) or a diesel particulate filter (“DPF”; see, e.g., U.S. Publ. No. 2010/0170230) to mechanically capture and oxidize soot. The soot accumulates on the filter surface, generates backpressure, and eventually needs to be burned off at elevated temperatures. Because most SCR catalysts are unable to withstand these high temperatures, separate systems are needed. However, because space is limited, designers would like to combine the SCR and soot oxidation functions into a single, compact after-treatment unit, which is typically the DPF (see, e.g., U.S. Publ. No. 2010/0180580). Such a combined system, known in the industry as an SCRF® catalyzed filter (product of Johnson Matthey), will have a catalyst that is exposed to soot, NH3, NO, and NO2. Although NO2 is beneficial for passively oxidizing soot that accumulates on the filter, it is normally consumed in the NH3-SCR reaction. Thus, depending on the configuration, the NH3-SCR and soot oxidation catalysts may need to compete for available NO2. With current SCR catalysts (e.g., transition metals dispersed in zeolites or mixed oxides such as the V2O5—WO3—TiO2 system), conversion of NO2 is much faster than the oxidation of soot by NO2. This difference in reactivity therefore limits the usefulness of known SCR catalysts for passive soot oxidation.
To minimize consumption of NO2 in the fast SCR reaction, the SCR catalyst can be coated on just the outlet channels of the DPF. This strategy was used in making the dual function catalytic filter described in PCT Int. Appl. No. WO 2012/166833. As shown in FIG. 1 of the '833 publication, the filter features a soot oxidation catalyst zone positioned closest to the direction of the exhaust gas flow, and an SCR catalyst zone positioned on the outlet side of the filter. Although the strategy is effective, it decreases the overall amount of SCR catalyst that can be deposited on the filter and may limit the degree of NOx reduction possible.
The industry would benefit from the availability of catalytic materials having attributes for both SCR and soot oxidation. A valuable material would have high density to enable high washcoat loadings, high SCR activity coupled with high thermal durability, and low selectivity for N2O formation, particularly in the presence of NO2. Ideally, the material would provide a desirable balance between NO2 consumption in the SCR reaction and NO2 availability for passive soot oxidation to facilitate the development of compact after-treatment systems for diesel exhaust.
As discussed earlier, improved SCR catalysts, particularly low-temperature NH3-SCR catalysts, are also needed. Catalysts that can retain or even improve NOx conversion activity when exposed to elevated temperatures are needed. Ideally, the catalysts would use valuable oxide components (e.g., niobia or tantala) more efficiently.